1. Field of the Invention
The invention relates to an image display apparatus, and more particularly to a structure of a phosphor substrate of the image display apparatus.
2. Description of the Related Art
There has been known a flat panel display of a system for emitting phosphor by electron beams emitted from electron-emitting devices. This type of the image display apparatus includes an electron source substrate having a lot of electron-emitting devices (a rear plate) and a phosphor substrate (face plate) on which phosphor films corresponding to the respective devices are disposed. A metal back as an anode electrode is disposed to the faceplate, and a high voltage of about, for example, 10 kV is applied to the metal back to accelerate emitted electrons.
When a discharge occurs between the face plate and the rear plate, a charge accumulated to the metal back flows to the rear plate. When the charge has a large current, there is a possibility that structural members such as the electron-emitting devices, wires, and the like are broken as well as drive circuits are broken. To cope with the above problem, it is preferable to increase the impedance of the face plate to restrict a current in a discharge operation. As an effective means for increasing it, there has been known an arrangement for dividing the metal back and connecting the divided metal backs to each other through high resistance members (refer to Japanese Patent Application Laid-Open No. 2006-173094).
However, in the arrangement for connecting the divided metal backs by the high resistance members, a voltage drop occurs due to a current flowing in the high resistance member in the discharge. Thus, a potential difference corresponding to the voltage drop is made between the adjacent metal backs or between the metal backs and the high resistance members. When the potential difference increases to a certain high level, a discharge occurs between the metal backs or between the metal backs and the high resistance members. Hereinafter, the phenomenon is called a collapse between the metal backs or between the metal backs and the high resistance members.
To prevent the collapse between the metal backs, it is sufficient to reduce an electric field strength applied between the adjacent metal backs and between the metal backs and the high resistance members in the discharge. Countermeasures to solve the above problems are as follows: (1) to reduce the amount of the voltage drop by reducing the resistance value of the high resistance members and (2) to increase the distance between the metal backs and distance between the metal backs and the high resistance members. The countermeasure (1) is not preferable because the decrease of the resistance value increases a discharged current. The countermeasure (2) restricts the shape and the size of the metal back.
The metal backs require a size that can cover the phosphor films. This is because a role of the metal backs is to apply an acceleration voltage to the phosphor films and to reflect light generated from the phosphor films. It is preferable that each of the phosphor films has a larger area from a viewpoint of improvement of luminance. Accordingly, it is desired that the metal backs that cover the phosphor films have also a large area.
From the above circumstances, a problem of trade-off occurs in that it is desired to increase the distance between the metal backs and the distance between the metal backs and the high resistance members as much as possible while increasing the areas of the metal backs.